In recent years, there has been a tendency of a rapid increase in electrically-powered equipment such as hybrid electric vehicle and electric power tools which require high rate discharge. As the electric source of the equipment, alkaline secondary batteries such as nickel metal-hydride batteries and nickel-cadmium batteries are widely used. As an active material for the nickel electrode, powder of high density nickel hydroxide {Ni(OH)2} is extracted via amine complex salt to reduce the self-discharge. Also, by adding solid solution of zinc (Zn) to nickel hydroxide, the formation of γ-NiOOH is inhibited. Furthermore, some modifications such as the charge efficiency improvement by inhibiting the generation of oxygen at the nickel electrode during charging through the forming of cobalt (Co) solid solution have been studied.
Also, for the secondary battery electrodes which use particles with poor conductivity such as metal oxides and hydroxides, it is a common method to improve the current collection function inside the battery plate by mixing the electrical conductive material such as a carbon powder or metal powder with the active material particle and then supporting the compound in the base plate. As for the nickel electrode for alkaline secondary battery, a method of building up an electrically conductive network is used by consisting of high-order cobalt compound in the aggregation of the active particle that consists of Ni(OH)2 and high-order nickel compound supported in the base plate in order to raise the active material packing density and secure the current collection function.
The electrically conductive network consisting of the aforementioned high-order cobalt compound can be formed by adding divalent cobalt compound such as cobalt monoxide {CoO} or cobalt hydroxide {Co(OH)2}, and the like to an active material powder that consists of nickel hydroxide, and forming a cobalt hydroxide layer on the surface of the core layer particle that mainly consists of Ni(OH)2, then assembling the alkaline secondary battery using a nickel plate filled with the active material powder on the base plate, and then generating a high-conductive and high-order cobalt compound with an oxidation of aforementioned cobalt compound whose oxidation number of cobalt is +2 by charging.
However, this method has a defect that the discharge capacity of the battery will decrease due to the increased amount of the discharge reserve production. In order to eliminate the defect, for example, a method has been proposed that forming a layer of Co(OH)2 on the surface of the core layer particle that mainly consists of nickel hydroxide, then oxidizing the active material powder with oxidant in the existence of alkaline solution, then turning the aforementioned cobalt compound on the surface layer to high-conductive and high-order cobalt compound before embedding it into a battery, and furthermore, increase the oxidation number of Ni higher than +2 by oxidizing a part of the Ni(OH)2 at the core layer. Although this method is able to decrease the discharge reserve, it has been quite difficult to increase the capacity of the nickel electrode itself or the charge efficiency.
As a reforming method of the active material for a nickel electrode (hereinafter referred simply as “active material”, too), one method is proposed by adding of alkaline metal (for example, Li) to the active material. It is reported that the capacity decay during the overdischarge can be inhibited by adding an alkaline solution containing Li+ to the active material which containing Zn and the like under the solid solution state, and then heating-up the system. In particular, an example is reported that adding the mixture solution of NaOH and LiOH to the mixture powder of hydrated nickel powder containing 2 mol % of zinc solid solution and Co(OH)2 powder and then heating-up at 100° C. under the atmosphere. (See Patent Reference 1 as an example.)    Patent Reference 1: Japanese Published Unexamined Application No. H8-148146 (page 4, paragraph 0029 and page 6, paragraph 0044)
However, in the method described in Patent Reference 1, as Li is not incorporated into Ni(OH)2 particles, it would not result in increase of Ni(OH)2 capacity per unit weight (mAh/g, hereinafter referred simply as “capacity density”). The reason maybe due to that Li is not captured in Ni(OH)2.
When it is charged up to the high oxidation state such as 3.5 oxidation number of nickel, in order to prevent positive electrode inflation by inhibiting generation of γ-NiOOH, for example, a nickel hydride battery equipped with a positive electrode which has an active material as a nickel hydroxide solid solution comprising at least one kind of transition metal and at least one kind of alkaline metal is proposed, and the preferable solid solution amount is 2 to 10 atomic % (atm %) of transition metal 5 to 10 atm % of alkaline metal against Ni of Ni(OH)2. (See Patent Reference 2 as an example.)    Patent Reference 2: Japanese Published Unexamined Application No. H10-289715 (page 3, paragraph 0017)
According to the description of Patent Reference 2, it is possible to improve the capacity density because it can be charged to the high oxidation number of Ni that is contained in the active material; however, it has a deficit such as an insufficient cycle characteristic.
Also, an active material for alkaline secondary battery that mainly consists of Ni(OH)2, having nickel hydroxide whose Ni valence is higher than divalent, having high-order cobalt compound containing the first alkaline cation on the surface of nickel hydroxide, and a positive electrode containing the nickel hydroxide whose valence is higher than the aforementioned divalent and containing the second alkaline cation has been proposed. (See Patent Reference 3 as an example.)    Patent Reference 3: Japanese Published Unexamined Application No. 20000-223119 (page 5, paragraph 0026-0027)
According to the description of Patent Reference 3, it can prevent peeling-off of the compound of high-order nickel hydroxide and high-order cobalt each other, and as the electrical conductivity of the positive electrode can be kept in a higher level, the utilization rate of the active material can be improved. For example, a type of nickel hydroxide compound that contains about 0.2 wt % of sodium ion and a type of nickel hydroxide compound that contains about 0.7 wt % of lithium ion are reported. However, in the method stated in Patent Reference 3, although the electrical conductibility would be increased, there is a tendency to deteriorate the charge/discharge cycle characteristic maybe because of the excessive lithium ion in the active material, and also as for cobalt compound, it only reports the one containing sodium ion but lithium ion.
Also, when the alkaline solution containing LiOH for the reaction bath is used in the process of oxidation treatment to oxidize a raw material powder consisting of Co(OH)2 and Ni(OH)2, which is reported that Li+ penetrates into the powder or adheres to the surface, resulting in obtaining a material with a high utilization rate. And then, it is reported that the concentration of alkaline solution is preferably 1 mol/l (1M/l) or higher. (See Patent Reference 4 as an example.)    Patent Reference 4: Japanese Published Unexamined Application No. 2002-110154 (page 5, paragraph 0032)
According to the method stated in Patent Reference 4, the utilization rate of the active material can be improved; however, in this method, even in normal charging condition, it has a tendency be overcharged, possibly due to the fact that the Li content ratio in the active material of nickel electrode is not controlled. Therefore, same as aforementioned Patent Reference 3, there is a tendency to deteriorate the charge/discharge cycle characteristic.
Also, at the nickel electrode that contains nickel hydroxide and cobalt hydroxide as an active material, when a lithium compound that consists of a salt of strong acid and strong base, or a salt of weak acid and strong base is added to the active material, lithium ion is incorporated into the nickel hydroxide crystal lattice, and by increasing the lattice defect, electron transfer would be accelerated and electrical conductibility would be improved. Also, the generation of the poor charge/discharge reversible γ-NiOOH would be inhibited, resulting in an improved utilization rate of the active material. (See Patent Reference 5 as an example.)    Patent Reference 5: Japanese Published Unexamined Application No. 2001-6679 (page 3, paragraph 0014)
However, in the Patent Reference 5, as an active material particle has not been previously oxidized and then Li might be difficult to be fixed in the active material particle during the addition of Li to the active material, when the utilization rate of the active material at 288 mAh per gram of the active material particle is 100%, the obtained highest utilization rate of the active material is just 100%, which is not necessarily satisfactory in the viewpoint of achieving a high capacity